Duplex metal article



Oct. 9, 1934. R BROWN 1,975,778

DUPLEX METAL ARTICLE Filed Oct. 20. 1935 INVENTOR 4 05567 hf 5E0 n/N BY A ORNEY Patented Oct. 9, 1934 DUPLEX METAL ARTICLE Robert H. Brown, New Kensington, ra, asslgnor, by mesne assignments, to Magnesium Development Corporation, a corporation of Delaware Application October 20, 1933, Serial No. 694,447

2 Claims.

This invention relates to duplex metal articles, and it is particularly concerned with the production of improved duplex metal comprising a base of a magnesium metal (magnesium or 5 a magnesium base alloy) provided on one or more of its surfaces with a coating consisting of a magnesium alloy.

It has heretofore been proposed to produce magnesium duplex metals of the type comprising a core of a magnesium alloy provided with a coating consisting of pure magnesium or a magnesium alloy more resistant to corrosive attack than the core metal. In some cases, however, such duplex metals possess the disadvantage that at any point where the core material is exposed in contact with the coating metal, as, for example, along the cut edge of the material, or where the coating has been perforated, an electrolytic type of corrision is set up which causes a preferential attack of the base metal with a consequent impairment of one or another of the structural properties, for example, strength, elongation, or yield point, which such duplex metals are designed to preserve. This preferential or selective attack on the core metal can occur when the core metal has an electrode potential greater than that of the coating metal, whereby voltaic action may be set up between the two metals in contact with the corroding medium, thus causing a flow of current which tends to dissolve the core metal. This type of corrosive action has militated seriously against the use of magnesium and certain of the magnesium alloys as a core or base metal for duplex metal articles.

It is an object of this invention to provide a duplex metal article consisting of a magnesium or a magnesium alloy base and a coating of an alloy consisting substantially of magnesium in which galvanic attack will be directed to the 40 coating alloy in preference to the underlying base metal. Another object of the invention is to provide a duplex metal article in which the base metal is magnesium or a magnesium alloy and the coating is an alloy consisting substantially of magnesium but having an electrode potential greater than that of the base metal. Another object is to provide a corrosion-resistant duplex metal article in which magnesium or various magnesium alloys may be used as a base metal, whereby any desired set of structural properties may be obtained.

This invention is predicated upon my discovery that by the addition of certain metals to magnesium the electrode potential of the magnesium may be substantially increased without seriously modifying the inherent properties of the magnesium itself, particularly its corrosion resistance. More particularly I have found that by the addition to magnesium of one or more of the class of 60 elements mercury, thallium, and lead, an alloy may be obtained having an electrode potential greater than that of most of the magnesium base alloys, while retaining the characteristics inherently valuable in the magnesiumwhen it is used 5 as the coating portion of duplex metal articles of the type described, so that a duplex metal article formed of such a magnesium alloy coating and a magnesium or magnesium base alloy base of the desired structural properties will have the substantially improved corrosion-resisting properties, the coating being attacked in preference to the base metal when exposed to corrosive agents in electrolytic contact therewith.

The class of elements above defined, namely, 7 mercury, thallium, and lead, are all possessed in this conn ction of the common characteristic of raising the electrode potential of the magnesium with which they are alloyed, but the reason why this particular group should have this particular efiectis not entirely clear, since, as will be immediately noted, these elements do not belong to any periodic classification, nor do they lie in any given position with respect to magnesium in the electromotive series of metals. These metals g5 raise the electrode potential of the magnesium in varying degree, depending, first, upon the particular metal or metals used, and, second, upon the particular amountswhich may be added to the magnesium. The choice of the proper magnesium alloy to be used as the coating material in combination with any given magnesium metal base depends upon a predetermined knowledge of the electrode potential of the base metal in question. The coating magnesium alloy containing one or more of the class of metals above described should have an electrode potential greater than that of the base metal. These different potentials are readily measured in accordance with methods long known in the art against a standard electrode. If the potential of the base metal is electronegative'with respect to the standard electrode, it is not necessary that the coating metal be electropositive with respect to the standard electrode but only that it be less electronegative than the base metal in order to be satisfactory for the purpose of my invention. Therefore, throughout this specification and claims, when I say that the electrode potential of the coating metal should be higher than that of the base metal, I mean that the coating metal should be more electropositive with respect to the standard electrode than is the base metal. While no fixed potential difference is required between the electrode potential of the base metal and that of the coating metal, a substantial protection may be given any base metal by a coating metal of higher electrode potential. It is generally desirable that there be a relatively wide difference in the electrode potential of the two metals, since I have found that with greater differences in potential there usually occurs less attack on the base metal,-and by suitable adjustment of the potential difference I have been able to afford the base metal complete protection.

A convenient standard electrode for measuring the potential difference mentioned may be readily selected from those now in regular use for similar measurements. For the purpose of this description of the invention, a calomel electrode has been selected as a standard. In measuring the electrode potential in question, the magnesium metal to be tested is made one element of on electrolytic cell; the calomel is made the other element. The electrolyte is a normal solution of sodium chloride containing 0.3 per cent by weight of hydrogen peroxide. The circuit is closed and the potential difference of the elements is measured ona potentiometer. The calomel electrode used is made up in the usual way of mercury in contact with mercurous chloride, and in contact with this a normal potassium chloride solution saturated with mercurous chloride.

The addition of one or more of the elementsnot deleteriouslyaffecting the desired potential properties and other desirable characteristics of the magnesium may be added. In general, the following amounts of the metalsof the class above described best serve the general purposes of this invention in producing the electrode potential desired without materially impairing other desirable properties of the magnesium alloy coating.

' Percent by Metal weight in V coating metal Mercury.-. 0.02 to 5.0 Thallium 0.01 to 2.0 Lead 0.5 to 15.0

The base or core metal that is to say,.the layer "or body of metal from which the duplex metal article of my invention derives its principal structural properties, may be any magnesium metal Composition-percentage by weight-balance magne- Anoy sium 4 aluminum, 0.4 manganese.

6.5 aluminum, 0.4 manganese, 0.75 zinc. 8.5 aluminum, 0.2 manganese, 0.5 zinc. 6.0 tin, 1.0 manganese.

4.0 aluminum, 0.6 manganese, 5.0 tin.

1 manganese, 5 tin, 4 zinc.

1.3 to 2.0 manganese.

Commercial magnesium.

In forming the duplex article of my invention, any of the known methods of forming such duplex metal articles may be used. One method which I have found particularly useful is to roll together the coating alloy and the base alloy in sheet or slab form. It is satisfactory to cast the base alloy in an ingot mold lined with the coating alloy. The article may then, if desired, be mechanically worked, as for example, by rolling or drawing or other suitable operation to form the particular type of duplex article desired, such as sheet, rod or other form of material. The working with intermediate heating incident to such forming operations tends to improve the bond between the coating metal and the base metal. Duplex material may also be produced by spraying or otherwise placing the coating alloy on the base metal. This method may be used to advantage where irregular shapes such as castings are to be coated.

The duplex articles of my invention may be provided on any one or all exposed surfaces with the protective coating alloy, as for example are shown in Figs. 1 and 2 of the drawing. Fig. 1 shows a duplex article comprising a base 1 of a magnesium metal provided on its top and bottom surfaces'with a magnesium alloy coating 2 and 2 respectively. In Fig. 2 is shown, in cross section, a duplex metal article in tubular form comprising abase 3 of a magnesium metal provided on its inner surface with a magnesium coating alloy 4. This type of material is useful, for example, in the manufacture of pipes used to conduct a material corrosive to the structural alloy.

I claim:

1. A duplex metal article consisting of a-base of a magnesium metal provided with a coating of an alloy consisting predominantly of magnesium andcontaining at least one of the class of metalsmercury, thallium, and lead, the alloy of the coating-having an electrode potential higher thanthat of the base metalas determined by reference to a standard electrode.

2. A- duplex metal article consisting of a base of a magnesium metal provided with a coating of an alloy consisting predominantly of magnesium and containing atleast one of the class of metals mercury, thallium, and lead in the following proportions: mercury 0.02 to 5.0 per cent, thallium 0.01 to 2.0 per cent, and lead 0.5 to 15.0 per cent, the alloy of the coating having an electrode potential higher than that of the base metal as determined by reference to a standard electrode. ROBERT H. BROWN. 

